Non-intrusive method to identify presence of nuclear materials using energetic prompt neutrons from photon-induced fission

ABSTRACT

Methods and systems for non-intrusively detecting the existence of fissile materials in a container via the measurement of energetic prompt neutrons are disclosed. The methods and systems use the unique nature of the prompt neutron energy spectrum from photo-fission arising from the emission of neutrons from almost fully accelerated fragments to unambiguously identify fissile material. The angular distribution of the prompt neutrons from photo-fission and the energy distribution correlated to neutron angle relative to the photon beam are used to distinguish odd-even from even-even nuclei undergoing photo-fission. The independence of the neutron yield curve (yield as a function of electron beam energy or photon energy) on neutron energy also is also used to distinguish photo-fission from other processes such as (γ, n). Different beam geometries are used to detect localized samples of fissile material and also fissile materials dispersed as small fragments or thin sheets over broad regions. These signals from photo-fission are unique and allow the detection of any material in the actinide region of the nuclear periodic table.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/270,865, entitled “NON-INTRUSIVE METHOD TO IDENTIFY PRESENCE OFNUCLEAR MATERIALS USING ENERGETIC PROMPT NEUTRONS FROM PHOTON-INDUCEDFISSION” filed on May 6, 2014, which is a continuation of U.S. patentapplication Ser. No. 12/139,050, entitled “NON-INTRUSIVE METHOD TOIDENTIFY PRESENCE OF NUCLEAR MATERIALS USING ENERGETIC PROMPT NEUTRONSFROM PHOTON-INDUCED FISSION” filed on Jun. 13, 2008, now issued as U.S.Pat. No. 8,718,219, which claims the benefit of and priority to U.S.Provisional Patent Application Ser. No. 60/944,009, entitled“NON-INTRUSIVE METHOD TO IDENTIFY PRESENCE OF NUCLEAR MATERIALS USINGENERGETIC PROMPT NEUTRONS FROM PHOTON INDUCED FISSION” filed on Jun. 14,2007 and U.S. Provisional Patent Application Ser. No. 60/971,638,entitled “NON-INTRUSIVE METHOD TO IDENTIFY PRESENCE OF NUCLEAR MATERIALSUSING ENERGETIC PROMPT NEUTRONS FROM PHOTON INDUCED FISSION ANDNEUTRON-INDUCED FISSION” filed on Sep. 12, 2007, and are all also herebyincorporated herein by reference; and

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Contract No.N66001-07-D-0025/Delivery Order No. 0001 awarded by the U.S. Navy. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

This disclosure relates to systems and methods for detecting thepresence of fissionable nuclear materials. The systems and methods makeuse of the distinctive signals provided by the energy and angulardistributions of the prompt neutrons produced in photon induced fissionof nuclei. They may be used to detect the presence of actinide nuclei(in particular those with Z greater than or equal to 89, that ofactinium). Some of these nuclei are classified as Special NuclearMaterials (SNM) and may be used in weapons of mass destruction such asnuclear explosives and in dirty bombs.

BACKGROUND

Illicit clandestine shipment of nuclear explosives, materials that canbe employed in the fabrication of nuclear explosives, and materials thatcan be employed in the fabrication of dirty bombs may constitute a majorthreat to the peace and security of the world. Such materials may besecreted and smuggled in cargo or other shipments in various containersincluding ordinary luggage, crates, vehicles, cargo containers, etc. byterrorists, potential terrorists, terrorist supporters, or others.Effective and efficient methods and systems are required for thereliable, non-intrusive, detection of such contraband materials in portsand in other cargo and shipping locations in order to reduce the risk ofsuccessful illicit shipments, without unduly impeding the worldwide flowof cargo in a manner that is disruptive of normal commerce. Accordingly,it is especially important that the detection methods not produce largenumbers of false positive detection events.

Passive detection methods, as for example gamma spectroscopy of naturaldecay, have not proven universally effective since many of the materialsof interest are not highly radioactive and are relatively easilyshielded. X-ray techniques do not readily distinguish betweenfissionable nuclear materials and innocuous high-Z materials like leador tungsten that may be legitimately present in cargo.

In addition to passive detection, several approaches to detection havebeen employed, attempted, or proposed using active techniques employingprobing beams.

In one such active technique, an external neutron source has been usedto detect fissionable nuclear materials by detecting induced fissionevents by the neutron multiplication effect of the fission events.However, it has been difficult to discriminate between the probingneutrons and the fission induced prompt neutrons, especially when theenergy of the probing neutrons is as high as the energy of the moreenergetic prompt neutrons from fission or when large containers areinvolved. Alternative techniques have induced fission events infissionable nuclear materials with pulsed external neutron sources, thendetecting delayed emission of neutrons by fission products, using timedelay, as a means of distinguishing the detected signal from the probingneutrons. This delayed neutron signal is a much weaker signal, and issubject to signal-to-noise ratio problems.

In other active techniques, gamma ray probe beams have been employed toinduce photofission (γ, f) of nuclear materials with detection ofneutrons resulting from the fission events. Scattered gamma rays fromthe probe beam as well as photo-neutrons (direct (γ, n) events resultingfrom interaction of the gamma probe beam with fissionable and/ornon-fissionable nuclei) induced by the probe beam contribute noise tothe detection of prompt neutrons from the fission events, contributingto unreliable or ambiguous detection. Photofission also results indelayed neutron production by the fission fragments, but as withneutron-induced fission, the delayed neutron signal is weaker anddetection suffers from noise problems.

It is therefore an object of this disclosure to provide improved systemsand methods for detecting fissionable nuclear material in an articlewith reduced error and ambiguity.

It is a further object of this disclosure to provide improved systemsand methods for detecting contraband fissionable nuclear materials byimproving discrimination of prompt fission neutrons in the presence ofnoise-contributing factors.

Another object of this disclosure is to provide systems and methods foranalyzing the energy or an energy spectrum of prompt fission neutrons todetect the presence of fissionable nuclear materials in an article.

A still further object of this disclosure is to provide systems andmethods for detecting an angular distribution of prompt fission neutronsto detect the presence of fissionable nuclear materials in an article.

Yet another object of this disclosure is to provide systems and methodsfor using an angular distribution of prompt fission neutrons and anenergy distribution of prompt fission neutrons to detect the presence offissionable nuclear materials in an article.

The objects set forth above as well as further and other objects andadvantages of the present disclosure are achieved by the embodimentsdescribed below.

SUMMARY OF THE INVENTION

A prompt neutron is a neutron emitted immediately after the fissionprocess; it is characterized by being emitted from a fission fragmentgenerally after the fragment has reached a significant fraction of itsfinal velocity, and thus may be referred to as a fully acceleratedfragment. The final velocity is imparted to the fragment by the strongCoulomb repulsion between the fission fragments. Some neutrons arisefrom photon induced fission at the point of scission (just as thefragments break apart) but these have been shown to be small in numbercompared to those emitted by the fragments in flight. There are alsodelayed neutrons that arise following the beta-decay of some of thefragments, but these are not considered herein since they are only asmall percentage of the neutrons emitted promptly and thus have anegligible effect on the practice of the methods disclosed herein. Oneof the advantages of utilizing prompt neutrons from photo-fission as adetection technique is that they are produced with approximately 200times the yield of delayed neutrons; this allows for higherprobabilities of detection, lower false positive rates, and faster scantimes.

The techniques and methods described herein make use of the boost invelocity (and thus energy) of a neutron that arises because the neutronis emitted from a rapidly moving nuclear fragment which has beenproduced by the (γ, f) process. This boost places the neutron in anenergy range that will allow for the unambiguous determination of thepresence of fissionable nuclei; this energy range is not possible fromother processes that could occur with other non-fissionable nuclei suchas direct neutron production by photons (γ, n). Additional features ofinterest are the nucleus-dependent angular distribution of the fragmentsin the photo-fission process and the prompt neutron energy distributionsat various angles. Thus the signature of photon-induced fission isunique. Also, by controlling the incident photon energy used to causethe fission, (γ, n) processes from other nuclei may be reduced inimportance or eliminated as a background. Since the process ofphoton-induced fission is ubiquitous with the actinides, these methodswill identify fissionable nuclear materials within a container, inparticular those which have Z equal to or greater than 89, that ofactinium.

This disclosure describes systems and methods for detecting fissilematerials by measuring prompt neutron energies and examining promptneutron energy spectra. The energy spectra of prompt neutrons thatoriginate from photo-fission are readily distinguishable from the energyspectra of neutrons that originate from other processes that may occurin non-fissile materials such as (γ, n). Neutrons at energies greaterthan E=E_(b)−E_(th), where E_(th) is the threshold for the (γ, n)process in relevant other heavy non-fissile elements and E_(b) is theendpoint energy of the incident bremsstrahlung photon beam (or theenergy of an incident monochromatic photon beam), indicate with noambiguity the presence of fissile material in the actinide region. Noother photon-induced process can generate neutrons with these energies.

Angular distributions of these neutrons reflect the angulardistributions of the fission fragments from which they arise:distributions deviating significantly from isotropy indicate thepresence of even-even nuclei while almost isotropic distributionsindicate the presence of odd-even or even-odd fissile species.(Hereinafter, in the interests of conciseness, “odd-even” shall denote anucleus with an odd number of nucleons, whether protons or neutrons, andthus the term hereinafter shall encompass both “odd-even” nuclei and“even-odd” nuclei.)

Comparison of the energy distribution of the prompt neutrons atdifferent angles also provides potentially useful information about thespecies present. If the energy distributions at different angles arenearly identical, the isotopes undergoing fission are odd-even; if theenergy distributions differ significantly at different angles, theisotopes undergoing photo-fission are even-even.

Another signature of photo-fission is the fact that the relative yieldof prompt neutrons at different neutron energies (i.e., the shape of theyield curve) does not depend upon the incident photon energy. This is incontrast to other processes such as (γ, n) where the relative yield ofneutrons at different energies is strongly dependent on incident photonenergy, particularly at the highest energies possible.

For a better understanding of the present disclosure, together withother and further objects thereof, reference is made to the accompanyingdrawings and detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show the fission fragment mass yields and associatedaverage neutron multiplicities as a function of fission fragment massresulting from neutron-induced fission of ²³⁵U and ²³⁹Pu, respectively;

FIG. 2 presents the time-of-flight (and energy) spectrum of neutrons forthe photo-fission of ²³²Th;

FIG. 3 shows the energy spectra of photo-neutrons produced by the (γ, n)process from gold for bremsstrahlung beams of 14.3 and 15.8 MeV endpointenergies;

FIGS. 4A and 4B show the photo-fission yield as a function ofbremsstrahlung endpoint energy in ²³⁵U and ²³⁹Pu, respectively;

FIGS. 5A, 5B, 5C, and 5D display the photon induced reaction crosssections for ²³⁹Pu for the (γ, Total), (γ, n), (γ, 2n) and (γ, f)processes, respectively;

FIG. 6 shows a schematic layout of a possible arrangement for oneembodiment of a system for detecting fissile materials in a container byanalyzing energetic prompt neutrons resulting from photon-inducedfission;

FIG. 7 shows the angular distribution of the neutrons emitted fromfission fragments from the photo-fission of ²³²Th and ²³⁸U;

FIGS. 8A and 8B are schematics showing two beam positions for detectingneutrons produced by fission fragments from photon-induced fission withthe angles for the detectors relative to the beam interchanged

FIG. 9 shows the angular distribution of the fission fragments from thephotofission of ²³²Th and ²³⁸U.; and

FIG. 10 shows the (n, f) cross section for ²³⁸U.

FIG. 11 shows pulse-shape discriminated energy spectra from HEU and Pbtargets irradiated with a 9 MeV bremsstrahlung beam, illustrating theseparation of photons from the neutron signal, and the separation ofprompt photo-fission neutrons from neutrons produced by the (γ, n)process in Pb.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Fission is a complex process that has been the subject of manytheoretical and experimental studies. (See generally Bohr and Mottelson,“Nuclear Structure”, 1998, World Scientific Publishing Co. Pte. Ltd.Singapore, and references therein). However, common empiricallyestablished features imply certain general regularities of the processindependent of nucleus or initiating particle.

When fission is spontaneous, initiated by low energy neutrons or by theabsorption of photons near the threshold for the (γ, f) process, thedominant mode of fission is the breaking apart of the nucleus into twofragments of unequal masses. These unequal masses are in the regions ofnucleon numbers 95 and 140 for ²³⁵U and in similar regions for otherfissionable nuclei. The fragments are accelerated by the strong Coulombrepulsion of their charges (Z₁, Z₂) and gain kinetic energy rangingapproximately from 160 to 180 MeV, depending on the nucleus undergoingfission. Most of this Coulomb energy is gained in approximately 10⁻²²sec as the fragments separate by several nuclear diameters. The finalfragment velocities correspond to kinetic energies of approximately 1MeV/nucleon for the light fragment and approximately 0.5 MeV/nucleon forthe heavy fragment. The rapidly moving fragments are generally excitedand emit prompt neutrons, mostly after they have gained most of thekinetic energy available from the Coulomb repulsion.

FIGS. 1A and 1B display the analysis by J. Terrell (“Neutron Yields fromIndividual Fission Fragments”, Physical Review, Vol. 127, Number 3, Aug.1, 1962, pages 880-904, and references therein) for the neutron inducedfission of ²³⁵U and ²³⁹Pu. These figures (which correspond to FIGS. 8and 9 in Terrell) display the asymmetric fragment mass distributionsfrom the neutron-induced fission and the average number of neutronsemitted from the heavy and light fragments, as a function of the mass ofthe fragments, for ²³⁵U and ²³⁹Pu. (The symbols ν, ν_(L) and ν_(H) inFIGS. 1A and 1B denote the average total number of neutrons emitted, theaverage neutrons emitted from the light fragment and the averageneutrons emitted from the heavy fragment, respectively, as a function offragment mass.)

Similar results have been obtained by Terrell for neutron-inducedfission of ²³³U and the spontaneous fission of ²⁵²Cf, showing thegenerality of the phenomena.

Many authors have studied the spontaneous fission of ²⁵²Cf, includingHarry R. Bowman, Stanley G. Thompson, J. C. D. Milton and J. Swiatecki:“Velocity and Angular Distributions of Prompt Neutrons from SpontaneousFission of ²⁵²Cf”, Phys. Rev., Volume 126, Number 6, Jun. 15, 1962 page2120-2136 and references therein. These authors were able to demonstrateby direct measurement that:

a) “The angular distribution (of the neutrons from the spontaneousfission of ²⁵²Cf) is strongly peaked in the direction of the fissionfragments. The relative intensities in the direction of the lightfragment, in the direction of the heavy fragment and at right angles areabout 9, 5 and 1 respectively”: and

b) “The broad features of the energy and angular distributions arereproduced by the assumption of isotropic evaporation (in the fragmentframe of reference) of the neutrons from fully accelerated fragments.”

While not the only important conclusions of the Terrell and Bowmanworks, those quoted and discussed here sustain the general descriptionof spontaneous fission or fission at low energies that is important tothe discussion herein.

The work of H. W. Schmitt, J. H. Neiler, and F. J. Walter, “FragmentEnergy Correlation Measurements for ²⁵²Cf Spontaneous Fission and ²³⁵UThermal-Neutron Fission”, Phys. Rev. Volume 141, Number 3, January 1966,Page 1146-1160, provides additional evidence of the features describedabove. They find that the average total fragment kinetic energies beforeneutron emission are 186.5±1.2 MeV for the spontaneous fission of ²⁵²Caand 171.9±1.4 MeV for neutron induced fission of ²³⁵U. The fragmentshave substantially all the kinetic energy available from the mutualCoulomb repulsion of the fragments.

Both the energy distribution and the angular distribution of theneutrons from fission fragments created by photon-induced fission arerelevant. The case of ²³²Th reported in C. P. Sargent, W. Bertozzi, P.T. Demos, J. L. Matthews and W. Turchinetz, “Prompt Neutrons fromThorium Photofission”, Physical Review, Volume 137, Number 1B, Jan. 11,1965, Pages B89-B101 is illustrative. These authors measured the spectraof neutrons from the photo-fission of ²³²Th at pairs of anglessimultaneously, 157 and 77 degrees relative to the photon beam, and 130and 50 degrees relative to the photon beam. They used bremsstrahlungphotons from electrons with kinetic energies of 6.75 and 7.75 MeV.Several subsidiary facts were important in their analysis:

1.) The (γ, n) threshold energy for ²³²Th is 6.438 MeV. Therefore, the(γ, n) process cannot contribute neutrons of energy greater than 0.31MeV and 1.31 MeV, respectively at the two energies of the electron beam,6.75 MeV and 7.75 MeV. Since these neutron energies are achieved only atthe end points of the respective bremsstrahlung spectra, there will notbe important contributions to the neutron spectra from the (γ, n)process even at neutron energies considerably lower than 0.31 or 1.31MeV, respectively; and

2.) The fission fragments in photo-fission, (γ, f), are known to havestrongly anisotropic angular distributions from ²³²Th. The distributionis peaked at 90 degrees to the incident photon beam, and the fragmentangular distribution is given by I=a+b sin²(θ), where θ is the anglebetween the incident photon beam direction and the fission fragmentdirection. The ratio b/a is considerably larger than 1 at the energiesdiscussed herein and remains larger than one even at incident photonenergies higher than 9 MeV. (E. J. Winhold, P. T. Demos and I. Halpern,Physical Review, 87, 1139 (1952): and, A. P. Berg, R. M. Bartholomew, F.Brown, L. Katz and S. B. Kowalski, Canadian Journal of Physics, 37, 1418(1959)). This fragment directionality provides the correlation betweenneutron angle and neutron energy that results from the velocity boost ifthe prompt neutrons are emitted from fragments that have their fullkinetic energy.

The results of analysis of the neutron energy spectra from ²³²Th (γ, f)are consistent with the following conclusions of Sargent et al:

1.) The fraction of the prompt neutrons that result from emission fromother than the fully accelerated fragments is 0.07±0.09;

2.) The prompt neutron angular distributions and energy distributionsare consistent with isotropic neutron evaporation with a thermal-typespectrum in the center of mass frame of reference of the movingfragments, where the fragments are moving with their fully acceleratedvelocities; and

3.) The energy spectrum of the neutrons in the center of mass frame ofreference is characterized by an average energy of 1.14±0.06 MeV. Thereare no significant components of temperature as high as or higher thanthis average energy. (That is, the ensuing Maxwellian energydistribution, were it applied to a fragment at rest in the laboratoryframe of reference without the kinematic boost from the motion of thephoto-fission fragments, would not yield many neutrons at the highenergies that result from applying the kinematic boost to neutronsemitted in the fragment frame of reference.) FIG. 7 presents angulardistributions of prompt neutrons from the fission fragments produced inthe (γ, f) process for incident photon energies near the threshold forthe (γ, f) process, for ²³²Th and ²³⁸U. It is taken from S. Nair, D. B.Gayther, B. H. Patrick and E. M. Bowey, Journal of Physics, G: NuclearPhysics, Vol 3, No. 7, 1977 (pp 1965-1978), who corroborate the relevant²³²Th results of Sargent et al. and also extend the results to thephoto-fission of ²³⁸U. These angular distributions are measured bydetectors which detect the fragments from neutron induced fission of²³⁸U. Therefore, they are an average over all the energies of theneutrons emitted from the photo-fission fragments convoluted with the(n, f) cross section. This emphasizes neutrons above approximately 1MeV, where the (n, f) cross section becomes large (See FIG. 10, whichpresents the (n, f) cross section for ²³⁸U. FIG. 10 is reproduced fromNational Nuclear Data Center, Brookhaven National Laboratory, ENDF,Evaluated Nuclear (reaction) Data File).

FIG. 9, also taken from Nair, presents the angular distributions of thefragments from the photo-fission for ²³²Th and ²³⁸U, for the sameincident photon energies as FIG. 7. The peaking of the neutrons from thephoto-fission fragments in the direction of the motion of the fissionfragments is clearly demonstrated by a visual comparison of FIG. 7 withFIG. 9. (The implications of the shape of the neutron angulardistribution are discussed below.)

FIG. 2, which is taken from FIG. 4 of the Sargent et al. reference,displays the time-of-flight (energy) spectrum of prompt neutrons fromphoto-fission of ²³²Th at 77 degrees with respect to the direction of anincident 7.75 MeV. photon beam. At the top of FIG. 2 is the promptneutron energy scale.

One outstanding feature of the neutron spectrum in FIG. 2 is thepresence of neutrons at very high energy compared to an evaporation(thermal) spectrum with an average energy of approximately 1.14 MeV, asreported by Sargent et. al. from their analysis of the energy andangular distributions of the prompt neutrons from photon induced fissionof ²³²Th. For example, the intensity at 6 MeV is considerable. Thepresence of a large number of neutrons at high energy results in partfrom the considerable boost in velocity transferred to the neutrons bythe moving fragments. For example, if the velocity of the fragmentcorresponds to a kinetic energy of 1 MeV/nucleon, then a 1 MeV neutronemitted in the fragment center of mass frame of reference in thedirection of fragment motion will have twice the velocity in thelaboratory frame of reference and a kinetic energy of 4 MeV. Thisfollows because the neutron velocity in the laboratory frame is the sumof the fragment velocity and the neutron velocity in the fragment frame.Since these are the same for the energies and directions considered inthis example, the velocity is doubled. The kinetic energy varies as thesquare of the velocity. Hence the neutrons with 1 MeV in the fragmentframe of reference have 4 MeV in the laboratory frame of reference. Moregenerally, if the fragment velocity is V and the co-directional neutronvelocity in the fragment frame is v, then the neutron velocity in thelaboratory frame is V+v. The kinetic energy of the neutron in thelaboratory frame is E=(m/2)(V²+2Vv+v²) orE=E_(f)(1+2(E_(n)/E_(f))^(0.5)+E_(n)/E_(f)) where E_(n) is the neutronkinetic energy in the fragment frame and E_(f) is the kinetic energy ofone nucleon of the fragment. Thus, in the above example, a neutronemitted in the fragment direction of motion at 2 MeV in the fragmentcenter of mass frame of reference will have a laboratory kinetic energyof 5.8 MeV.

Energy conservation in the direct (γ, n) neutron production process doesnot allow the production of neutrons with an energy aboveE=E_(b)−E_(th), where E_(b) is the bremsstrahlung endpoint energy of theincident photon beam and E_(th) is the (γ, n) threshold energy forproducing neutrons from other relevant heavy elements. Therefore,detecting neutrons with energies above this value is definitive evidenceof the presence of fission.

Since the (γ,n) threshold of ²³²Th is 6.438 MeV, a neutron energy of 6MeV will not be possible from (γ, n) until the bremsstrahlung endpointreaches 12.438 MeV. Also, even when the bremsstrahlung endpoint reachesthat value, neutrons from the (γ, n) process will be very small innumber because they can only be produced by the few photons at thebremsstrahlung endpoint energy.

These energetic considerations apply in a similar manner for allfissionable nuclear materials, in particular for those with Z≥89, theregion of the actinides. In addition, and most importantly, most heavyelements such as Bi, Pb, W, Ta, etc. have isotopes with (γ, n)thresholds at or above 6.5 MeV. Therefore, finding neutrons withenergies above E=E_(b)−E_(th) where E_(th) is in the range of 6 MeVconstitutes a very definitive test for the presence of fissile material.

Another test to verify that the detected neutrons result fromphoto-fission is the sensitivity of the yield of neutrons at energiesabove E=E_(b)−E_(th) to a modest increase in incident photon energy. Inparticular, measuring the increase in yield relative to the yield ofneutrons below this energy is significant. The increase or relativeincrease in neutron yield is not substantial when the neutrons areemitted from photo-fission fission fragments because energeticconsiderations independent of the exact incident photon energy, such asthe boost in velocity from fission fragment motion, are most importantin determining the yield.

FIG. 3 displays spectra of (γ, n) neutrons for gold. (It is FIG. 2 fromW. Bertozzi, F. R. Paolini and C. P. Sargent, “Time-of-FlightMeasurements of Photoneutron Energy Spectra”, Physical Review, 119, 790(1958)). FIG. 3 illustrates how the nature of the (γ, n) process causesneutrons produced by that process to be concentrated mostly at lowenergies. The data in FIG. 3 are normalized to yield the same number ofneutrons from the (γ, n) process in a reference target of ²D withneutron energies E_(n)>1.4 MeV. Because photon and neutron energy areuniquely related in the (γ, n) process in ²D, this normalization allowsthe formation of the difference photon spectrum (the difference betweenthe high energy (15.8 MeV) bremsstrahlung spectrum and the low energy(14.3 MeV) bremsstrahlung spectrum), which corresponds to a broad bandof photons centered at approximately 14.5 MeV and with approximately a 2MeV half width. That is, the neutron energy spectrum produced by thedifference in the neutron energy spectra at the two energies in FIG. 3corresponds to photo neutrons produced by photons in the above energyband centered at approximately 14.5 MeV. FIG. 3 confirms that, becauseneutrons produced by the (γ, n) process are concentrated mostly at lowenergies, the contamination of a photo-fission spectrum by neutrons fromthe (γ, n) process is expected to be low at higher neutron energies,even when one looks at neutrons at energies below the E=E_(b)−E_(th)cutoff established by the strict application of energy conservation.

The spectra in FIG. 3 show the rapid, almost exponential decrease ofneutrons from the (γ, n) process with increasing neutron energy, incontrast to the neutron spectrum from the photo-fission (γ, f) of ²³²That 7.75 MeV bremsstrahlung energy as shown in FIG. 2. For gold theneutron spectrum from (γ, n) is nonexistent with if the bremsstrahlungspectrum endpoint is 7.75 MeV, since the (γ, n) threshold, E_(th), isabove 8 MeV. Even with a 12 MeV bremsstrahlung endpoint, the highestneutron energy from (γ, n) in gold would be less than 4 MeV, andneutrons in this energy range from (γ, n) would not be numerous becausethey would correspond to photons at the endpoint of the bremsstrahlungspectrum. The neutron yield from (γ, f) in ²³²Th is very large at 12 MeVbremsstrahlung for neutron energies above 6 MeV.

Table 1 gives the (γ, f) and the (γ, n) thresholds (in MeV) for sometypical nuclei in the actinide region. The (γ, f) thresholds are from H.W. Koch, “Experimental Photo-Fission Thresholds in ²³⁵U, ²³⁸U, ²³³U,²³⁹Pu and ²³²Th”, Physical Review, 77, 329-336 (1950). The (γ, n)threshold of ²⁰⁷Pb is also listed, as it is a component in natural leadmaterial that may be used as a shield against detection of fissilematerials. The table shows the maximum neutron energy available from the(γ, n) process for bremsstrahlung end point energies up to 11 MeV,including for ²⁰⁷Pb. This energy is to be compared to the spectrum inFIG. 3 showing many neutrons with energies in excess of 6 MeV from ²³²Thphoto-fission using bremsstrahlung of only 7.75 MeV. Even with an 11 MeVbremsstrahlung energy there are no neutrons above 5.7 MeV from anynucleus, and no neutrons above 4.26 from ²⁰⁷Pb, and those at or nearthese energies would be very few in number because they correspond tothe photons at or near the end-point energy of the bremsstrahlungspectrum. It should be noted that the (γ, f) process increases inimportance as the bremsstrahlung endpoint energy increases from 6 to 11MeV because of the increasing cross section with energy and because ofthe increasing number of photons in the bremsstrahlung spectrum at lowerphoton energies where the (γ, f) cross section is sizable. The (γ, f)thresholds are almost all lower than the (γ, n) thresholds, and are allsignificantly lower than the (γ, n) threshold for ²⁰⁷Pb.

TABLE 1 Maximum Neutron Energies from (γ, n) for Selected BremsstrahlungEnergies and Isotopes. Maximum (γ, n) Neutron Energy (MeV) (γ, f)Threshold (γ, n) Threshold Bremsstrahlung γ Endpoint Energy, E_(b) (MeV)Element (MeV) (MeV) 6 7 8 9 10 11 ²³²Th 5.40 ± 0.22 6.438 — 0.56 1.562.56 3.56 4.56 ²³³U 5.18 ± 0.27 5.759 0.24 1.24 2.24 3.34 4.34 5.34 ²³⁵U5.31 ± 0.27 5.298 0.70 1.70 2.70 3.70 4.70 5.70 ²³⁸U 5.08 ± 0.15 6.154 —0.85 1.85 2.85 3.85 4.85 ²³⁹Pu 5.31 ± 0.25 5.647 0.35 1.35 2.35 3.354.35 5.35 ²⁰⁷Pb — 6.738 — 0.26 1.26 2.26 3.26 4.26

The data in Table 1 indicates how the yield of neutrons above aspecified energy would change as the bremsstrahlung endpoint energy ischanged. For ²⁰⁷Pb, Table 1 indicates, there would be no neutron yieldabove 4 MeV until the electron beam energy exceeded approximately 11MeV. (For gold, as discussed above in connection with FIG. 3, theelectron beam energy would have to exceed 12 MeV to provide a neutronyield above 4 MeV.) However, the yield of neutrons above 4 MeV for theactinides would be a strongly increasing function of electron beamenergy starting below 6 MeV electron beam energy since the low (γ, f)threshold allows the photo-fission process to grow rapidly as more andmore photons are available for photo-fission, all of them producing aneutron spectrum independent of photon energy and strongly populatingthe selected region of neutron energy (above 4 MeV for example). The (γ,n) process in the actinide examples shown in Table 1 or in other heavymetals such as ²⁰⁷Pb would not be a significant component of the totalyield until the electron beam energy is well above 10 MeV since theprocess involves only the photons near the bremsstrahlung endpoint,E_(b).

An additional point, which will be discussed further below, is that thephoto-fission cross section is larger than the (γ, n) cross section overmost photon energies by a considerable amount, as shown in FIGS. 5B and5D. The neutrons from (γ, f) will dominate (γ, n) in most situationssimply on the basis of the cross sections, aside from the other featuresdiscussed herein.

The data in Table 1 is based upon continuous bremsstrahlung spectra withspecific endpoint energies, but a similar discussion applies tomonochromatic photon beams. The neutron energy spectra fromphoto-fission retains the same dependence on neutron energy fordifferent photon energies, but the total yield is modulated formonochromatic photons only by the cross section for (γ, f) at thespecific photon energy. In contrast, the total yield for neutronproduction from a bremsstrahlung beam is modulated by the convolution ofthe bremsstrahlung spectrum with the (γ, f) cross section. The maximumneutron energy from (γ, n) dictated by energy conservationconsiderations for monochromatic incident photons follows just asdiscussed above.

Other energies than 4 MeV could be used as the “trigger” or cutoff fordefining the presence of fissionable nuclear material. That is, for anyspecific electron beam energy, a “trigger” energy can be selected suchthat the presence of neutrons with an energy above that “trigger” energywill be energetically impossible for the (γ, n) process in relevantheavy materials such as ²⁰⁷Pb and therefore any neutrons detected couldonly originate from the photo-fission process in an actinide. The datain FIG. 2 show that there are many neutrons above 6 MeV from the (γ, f)process, and hence 6 MeV could be selected as a “trigger” energy. Other“trigger” energies are possible also; the choice is dependent on factorssuch as the speed of detection that is desirable, the false positivesthat are to be allowed, and the efficiency of detection that is desired.

In addition, the choice may be dictated by the specific nature of thecargo in a container; if the cargo is made of materials with high (γ, n)thresholds, such as copper, aluminum, steel or oxygen, then a lowertrigger could be selected.

Conversely, hydrogenous material that naturally contains a smallpercentage of deuterium may be of concern because of its low thresholdfor the (γ, n) process, 2.2 MeV. However, because the energy release isshared almost equally by the neutron and proton, the maximum neutronenergy is given by E=(E_(b)−2.2)/2 MeV and, for the example of anelectron beam energy of 10 MeV, the maximum neutron energy isapproximately 3.9 MeV and a 9.2 Mev photon results in a neutron energyof 3.5 MeV. Thus, a higher trigger may be appropriate

A more important concern may be ⁹Be. It has a low (γ, n) threshold ofonly approximately 1.6 MeV and the energy sharing results in a neutronthat has most of the available energy, E=(8/9)(E_(b)−1.6) MeV is themaximum neutron energy available. For the example of E_(b)=10 MeV, themaximum neutron energy is approximately 7.5 MeV. This high energy couldpresent a serious background. However, one could distinguish neutronsfrom actinide photo-fission from neutrons from the (γ, n) process in ⁹Beby taking advantage of the fact that the (γ, n) process follows thestrict rule for conservation of energy, so that E=(8/9)(E_(b)−1.6)defines the maximum neutron energy possible, while the photo-fissionprocess has a neutron energy spectrum largely independent of the photonenergy in the energy region under discussion, E_(b) less thanapproximately 15 MeV. Therefore, neutrons at an energy greater thanE=(8/9)(E_(b)−1.6), where E_(b) is the photon beam energy orbremsstrahlung endpoint energy, is proof of a fissile material. AtE_(b)=10 MeV, the presence of neutrons above approximately 7.5 MeV wouldbe proof. At E_(b)=8 MeV , neutrons above 5.7 MeV would be proof. Also,the prompt neutron energy spectrum is independent of the photon energywhile the (γ, n) process in ⁹Be produces a neutron spectrum that isstrongly dependent on photon energy. This difference also permitsdistinguishing the presence of a fissionable element from the presenceof ⁹Be.

However, if there were concern that this measurement could not bereliably made, further steps could be taken. Operating at E_(b)=10 MeV,the maximum neutron energy from beryllium (γ, n) is approximately 7.5MeV. By reducing the beam energy to 8 MeV, for example, the maximumenergy neutron from beryllium (γ, n) would be reduced to 5.6 MeV but thephoto-fission neutron energy distribution would be unchanged. If thereare neutrons above 5.6 MeV the process is unquestionably photon inducedfission. If there remains any doubt that neutrons are from fission, thephoton beam energy can be further reduced. For example at 5 MeV photonor bremsstrahlung beam energy there will be little or no photo-fission.But beryllium (γ, n) will produce neutrons of up to approximately 3 MeVat that photon beam energy. The presence of these neutrons will clearlyestablish the presence of beryllium. From the yield of these neutrons,the contributions from beryllium to higher neutron energies when higherphoton energies are used can be calculated, the neutron energydistribution from beryllium removed, and the remaining spectrum analyzedfor the presence of actinide neutrons.

Fortunately, ⁹Be is almost unique in this category. There are a fewother nuclei with relatively low (γ, n) thresholds; ⁶Li, ¹³C, ¹⁷O and¹⁴⁹Sm are notable among these with thresholds of 5.66, 4.95, 4.14 and5.87 MeV, respectively. The same procedures outlined above can be usedto eliminate these sources as contributors masking fissionable nuclei.

FIGS. 4A and 4B, from H. W. Koch, “Experimental Photo-Fission Thresholdsin ²³⁵U, ²³⁸U, ²³³U, ²³⁹Pu and ²³²Th”, Physical Review, 77, 329-336(1950), FIGS. 4 and 5, display the yield of fission fragments as afunction of bremsstrahlung endpoint energy (“Peak Spectrum Energies”)for two isotopes, ²³⁵U (FIG. 4A) and ²³⁹Pu (FIG. 4B). These illustratethe rapid increase of the fission yield as a function of the energy ofthe electron beam used to produce bremsstrahlung. FIG. 4A also shows thedominance of the ²³⁵U contribution over the impurities of ²³⁸U in theenriched uranium sample. These data are based upon the detection of theactual photo-fission fragments. The yield of prompt neutrons followsapproximately the same yield curve, since neutron emission in thephoto-fission process is not dependent on the photon energy in theregions of interest below the Giant Electric Dipole Resonance atapproximately 12 to 13 MeV photon energy. The emission of neutrons fromthe fragments is determined by the complex dynamics, discussed earlier,of splitting the fissioning nucleus into two fragments.

As a result, the shape of the yield curve of prompt neutrons of a givenenergy as a function of bremsstrahlung energy will be essentiallyindependent of the neutron energy. That is, the yield curve for 6 MeVneutrons will have the same dependence on bremsstrahlung endpoint energyas the yield curve for 2 MeV, 3 MeV, 4 MeV and etc. neutrons. This is incontrast with the yield curves for neutrons from the (γ, n) process,which will start at the endpoint energy given by E_(b)=E_(th)+E_(n),where E_(n) is the neutron energy that is desired. They are thusdisplaced from the (γ, n) threshold energy, E_(th), by the neutronenergy, in contrast to the yield curves for (γ, f). This is a powerfulsignature that the neutrons detected are from photo-fission rather thanfrom (γ, n).

FIGS. 5A through 5D display the photon induced reaction cross sectionsfor ²³⁹Pu. They are taken from FIG. 7 of B. L. Berman, J. T. Caldwell,E. J. Dowdy, S. S. Dietrich, P. Meyer, and R. A. Alvarez, “Photofissionand photoneutron cross sections and photofission neutron multiplicitiesfor ²³³U, ²³⁴U, ²³⁷Np and ²³⁹Pu”, Physical Review C Volume 34, Number 6,2201-2214 (1986). FIG. 5A shows the total photon absorption crosssection. FIG. 5B shows the partial cross section for (γ, 1n), singleneutron emission. FIG. 5C shows the partial cross section for (γ, 2n),double neutron emission. FIG. 5D shows the partial cross section for (γ,f), photo-fission.

The photo-fission cross section (FIG. 5D) is larger than the (γ, n)cross section (FIG. 5B) over most photon energies by a considerableamount. This displays the feature common to the actinides thatphoto-fission is a strong and often dominant process from the (γ, f)threshold throughout much of the Giant Electric Dipole Resonance. Giventhat the prompt neutron multiplicities from photo-fission range fromapproximately 2.5 to more than 3, prompt neutrons from the photo-fissionprocess will dominate the incident photon reaction channel by a largefactor at all neutron energies. This feature facilitates identifying thepresence of actinide fissionable material despite the potential presenceof other heavy elements such as Pb, even without considering theenergy-conservation constraints on neutron energy. The photon absorptionprocess in most heavier nuclei is dominated by neutron emission, and thetotal yield is governed by the giant dipole sum rule that the integratedcross section is proportional to NZ/A, which is a slowly varyingfunction. Since the location of the giant dipole resonance in energyalso is a slowly varying function of nuclear mass, a yield of promptneutrons from the photo-fission process that is 2.5 to 3 times theexpected neutron yield from (γ, n) is a signal of photo-fission in thatthe (γ, f) neutron yield alone will produce a markedly higher photonabsorption cross section than would (γ, n) for a given quantity of heavymaterial. That is, measuring the yield of neutrons per heavy nucleus perphoton permits identifying photo-fission as present, if the quantity ofheavy material present can be determined by measuring localized densityby other methods.

The angular distribution of the prompt neutrons and the relationship ofthe neutron energy to the fragment angular distribution also aresignatures of fissile material and the photofission process, and can beused in detection schemes.

The fragment angular distributions are not as distinct for odd-evennuclei as for even-even nuclei, in part because of the high populationof spin states. Odd-even nuclei angular distributions are almostisotropic as reported by L. P. Geraldo, “Angular Distribution of thePhotofission Fragments of ²³⁷Np at Threshold Energy”, Journal of PhysicsG: Nuclear Physics, 12 1423-1431 (1986), which shows angular anisotropyof approximately 10% at 5.6 MeV, 6% at 6.61 MeV and ˜2% at 8.61 MeV.These results are very much in contrast with the large anisotropy forfragments from the photo-fission of even-even nuclei where ground statespins are zero. Thus, once actinide photo-fission is detected, a nearlyisotropic neutron angular distribution is an indicator of an odd-evenfissile species such as ²³⁵U, ²³⁷Np and ²³⁹Pu. A strongly anisotropicneutron angular distribution would indicate an even-even fissile speciessuch as ²³²Th and ²³⁸U. (See S. Nair, D. B. Gayther, B. H. Patrick andE. M. Bowey, Journal of Physics, G: Nuclear Physics, Vol 3, No. 7, 1977(pp 1965-1978) and references therein, for example.)

The energy distributions of the neutrons at various angles arethemselves indicators of the fragment anisotropy, and thus of the typeof nucleus. This fact was used in the analysis of the work by Sargent etal, discussed above. If the fragments are strongly anisotropic(even-even fissile species), then the energy spectra of the neutronswill show distinct differences at different directions with respect tothe photon beam. As an example, if the fragments are strongly peaked at90 degrees with respect to the photon beam, then the neutron spectrum at90 degrees will exhibit to a different degree the boost in velocity dueto the velocity of the fragments than the neutron spectrum at anglesnear 180 degrees or 0 degrees to the photon beam. However, if thefragment angular distribution is nearly isotropic (odd-even fissilespecies), then the energy distribution of the neutrons will be the sameat all angles. In both situations, the higher energies reflect themotion of the fragments, but the contrast in the energy distribution ofthe neutrons at different angles will reflect the fragment anisotropywith angle.

The fragment angular distributions dominate the neutron angulardistributions and the neutron energy distributions as a function ofangle. The results of E. J. Winhold, P. T. Demos and I. Halpern,Physical Review, 87, 1139 (1952); E. J. Winhold and I. Halpern, PhysicalReview, 103, 990-1000 (1956); and, A. P. Berg, R. M. Bartholomew, F.Brown, L. Katz and S. B. Kowalski, Canadian Journal of Physics, 37, 1418(1959) show the fragment angular distributions for various isotopes. Thefollowing abstract from Berg et al. is offered as a summary of the datain that paper:

-   -   Angular distributions of photofission fragments relative to the        photon beam have been measured as a function of maximum        bremsstrahlung energy in the range 6-20 Mev. The nuclides U-233,        U-235, Np-237, Pu-239 and Am-241 give an isotropic distribution        at all energies studied. The nuclides Th-232, U-234, U-236,        U-238, and Pu-240 give anisotropic distributions which can be        described by an equation of the form W(Θ)=1+α sin²Θ, where θ is        the angle between fragment and beam. The degree of anisotropy is        large at low energy and falls rapidly as the energy is        increased. At a given energy Th-232 has the greatest degree of        anisotropy and Pu-240 the least.

The result quoted in the abstract is in basic agreement with that of theother papers referred to herein. In addition, some greater detail aboutthe results from Berg et al. is shown in the two tables taken from thatreference:

TABLE 2 Angular Distributions (from Berg, et al. Table I) Angulardistributions Ratio, counts at 90°/counts at 0°* Nuclide E₀† = 6.0 E₀ =6.5 E₀ = 8.0 E₀ = 10.0 E₀ = 20.0 U-233 1.048 ± 0.07 1.032 ± 0.04 0.994 ±0.03 U-235 1.024 ± 0.05 Np-237 1.024 ± 0.10 Pu-239‡ 1.034 ± 0.927 ±1.002 ± 0.06 1.013 ± 0.05 0.952 ± 0.03 0.26 0.12 Am-241 0.958 ± 0.07*The ratio is the number of counts observed at 90° per unit X-ray dosedivided by the number observed at 0° for the same dose. †E₀ is themaximum energy in million electron volts of the bremsstrahlung spectrum.‡The 45°/0° ratio at E₀ = 6.5 Mev was 1.09 ± 0.23.

TABLE 3 Corrected Values of α (from Berg, et al. Table VI) Correctedvalues of α in W(θ) = 1 + α sin² θ E₀ Th-232 U-238 U-236 U-234 Pu-2406.0 6.6 ± 2   6.0 ± 2.3 6.3 6.7 ± 1.1 6.5 >25 4.4 ± 1.0 2.1 ± 0.4 2.3 ±0.6 0.65 ± 0.20 7.0 11.0 ± 0.8  2.05 ± 0.24 1.33 ± 0.17 0.90 ± 0.16 0.49± 0.12 7.5 10.3 ± 1.6  8.0 4.9 ± 0.6 1.3 ± 0.1 0.79 ± 0.09 0.44 ± 0.080.29 ± 0.07 9.0 2.8 ± 0.4 0.51 ± 0.07 9.4 0.44 ± 0.04 10.0 1.61 ± 0.120.41 ± 0.05 0.32 ± 0.06 0.17 ± 0.07 14.0 0.46 ± 0.09 0.09 ± 0.04 0.04 ±0.03 15.0  0.02 ± 0.04*  0.01 ± 0.03* 20.0 0.14 ± 0.06 0.05 ± 0.03*These values, which do not differ from zero, have not been correctedfor isotopic composition.

Table 2 (“Angular Distributions . . . ”) shows that the ratio of eventsat 90 degrees to those at 0 degrees for the photo-fission of theodd-even isotopes shown is approximately equal to 1 over the energyrange of the bremsstrahlung endpoints shown in the table. Thus, thevalue of b/a discussed earlier is 0 and the angular distribution isisotropic. Table 3 (“Corrected values . . . ”) shows the fit to thenormalized form of the angular distribution as exhibited in the tablealso as a function of bremsstrahlung endpoint. The derived angulardistributions are clearly anisotropic. From these data, the quotedabstract, and the theoretical basis referred to in the referencesherein, the generalization is accurate; the odd-even actinides undergoisotropic photo-fission while the even-even actinides undergoanisotropic photo-fission. In particular, the result is experimentallydemonstrated for the isotopes most likely to be used for a nuclearweapon, ²³⁵U, ²³⁹Pu and ²³⁷Np. These will undergo isotropicphoto-fission, in contrast to ²³⁸U, ²³²Th and the other even-evenisotopes that were measured.

FIG. 9, which displays the fission fragment angular distributions fromphoto-fission of an even-even nucleus, and FIG. 7, which displays theangular distributions of the prompt neutrons emitted from thosefragments, demonstrate the general peaking of the fragment and neutronangular distributions at 90 degrees relative to the photon beam. Theneutron yield at 150 degrees is approximately 20% less than that at 90degrees and the shape of the distribution is approximately symmetricabout 90 degrees. The fragment angular distributions show a largeranisotropy as expected because the neutron distributions are produced byfolding the isotropic angular distributions in the fragment center ofmass with the fragment distributions in angle. In contrast to thesedistributions, the isotopes ²³⁵U, ²³⁹Pu and ²³⁷Np (not shown), which maybe used in the manufacture of weapons, display for the most partisotropic angular distributions of the photo-fission fragments asdiscussed above and the resulting angular distributions of the promptneutrons also are isotropic.

One embodiment of a detector system to carry out the methods describedherein requires a source of photons with energy capable of exceeding the(γ, f) threshold and a detector for neutrons. The photons may bemonochromatic, may be produced by a source capable of variable energy,or may be distributed over a broad range of energy with a gooddefinition of the highest energy possible, such as an electron-generatedbremsstrahlung spectrum in accordance with the discussion above. When anaccelerator is used to provide the electrons, the electron acceleratormay have the capability to vary the energy of the electron beam frombelow the fission barrier (threshold) to higher energies in order toexploit all the modalities discussed above.

Any neutron detector that is capable of distinguishing neutron energy isappropriate. A detector that takes advantage of energy deposition, suchas proton recoil from neutron elastic scattering in a hydrogenousscintillator, is a possible choice. A detector that measures a reactionenergy induced by the neutron is another possible choice. A method ofmeasuring neutron energy by time of flight is also an appropriatedetection scheme. The energy resolution required for such detectionmethods will have to be sufficient to eliminate neutrons from the (γ, n)process in materials other than actinides, as discussed above.

Because the contamination of non-actinide (γ, n) can be controlled andrendered small by the choice of incident photon energy (orbremsstrahlung endpoint) and neutron energy measured, the resolutionrequired is well within a number of measurement techniques. Specificresolutions required may depend in detail on the particular situationunder consideration, but resolutions of approximately 0.5-0.75 MeV at 4to 6 MeV neutron energy may be adequate.

A detection method may be required to operate in a possible flux ofphotons in some embodiments, these photons being produced by scatteringfrom the material under study in the direction of the detectors. Photonsmay also be produced by natural radioactivity and cosmic rays.Therefore, the neutron detectors may have to be shielded using passiveand active shielding techniques.

In addition, as a consequence of the above, a neutron detector may berequired to distinguish between photons and neutrons. This can beaccommodated by the reaction process used, the time of flight of thephotons compared to neutrons and by the ability of the detector todistinguish between the deposition of energy by heavy particles (e.g.,neutrons) compared to electrons. Organic and inorganic scintillatorsthat have different decay times according to the density of ionizationproduced by the passage of a charged particle may be suitable.Separation of photons from neutrons may be achieved in suchscintillators utilizing signal processing techniques that exploit thesedifferent charged particle responses.

FIG. 11 demonstrates the energy separation of prompt neutrons fromphoton induced fission from neutrons produced from the (γ, n) reaction,when incident photon energies below 9 MeV are used. That Figure wasobtained using 9 MeV bremsstrahlung beams produced at the CW S-DALINACat the Technical University of Darmstadt. The spectra from Pb and highlyenriched uranium (HEU) targets shown in FIG. 11 were obtained using thetechnique proposed for the CAARS PNPF module. An organic liquidscintillator was used to determine the energy deposited in the detectorand to separate photon and neutron events. The lighter data pointsrepresent events from Pb (which as discussed above has a (γ, n)threshold of approximately 6.5 MeV for the ²⁰⁷Pb isotope) and the darkerrepresent events from HEU (which has a photo-fission threshold of 5.5MeV). The neutron events are grouped in the lower portion of the figureand are clearly differentiated from the photon events above. Within theneutron events, the box shows where the neutrons from promptphoto-fission in HEU appear unambiguously. A neutron signal in thisregion is an unambiguous signal of an actinide photo-fission event.

One exemplary embodiment of a system 600 for detecting fissile materialsin a container by analyzing energetic prompt neutrons resulting fromphoton-induced fission is illustrated in FIG. 6. An electron beam 602 ofenergy E_(b) is generated by an electron accelerator 601. The electronbeam 602 makes bremsstrahlung radiation photons when it strikes abremsstrahlung target 603 (also called the radiator). The electronaccelerator 601 and radiator 603 optionally may be replaced by a sourceof monochromatic or nearly monochromatic photons. The optionalcollimator 604 collimates the bremsstrahlung radiation. A shield 605 mayenclose the bremsstrahlung target 603 and electron accelerator 601. Thephoton beam 607 is directed onto a container 606 which is to be analyzedand which may contain fissile material 608. The distances of the fissilematerial 608 from (for example) three of the walls of container 606 aredesignated as x, y and z. A photon detector 609 placed after thecontainer 606 optionally may be used to monitor the transmitted photonflux of photon beam 607. Detectors 610, 611, 612, and 613 may be placedat locations around the container 606 at approximately 90 degrees and atconvenient back angles with respect to the collimated photon beam 607.The number and location of the detectors may be varied from that shownin FIG. 6 according to the principles and methods discussed above. Inthe illustrated embodiment, the detectors 610 and 611 may be placed atknown distances L610 and L611 from the container 606 walls. Thedetectors 610, 611, 612, and 613 optionally may be surrounded byshielding (not shown) and by anti-coincidence counting systems (notshown) if desired. The detectors 610, 611, 612, and 613 themselves maybe sensitive to neutron energy or they may be part of a system (such asone utilizing time-of-flight) that will provide a neutron energy foreach detected neutron event. A beam dump 614 may be used to absorb theremaining photon flux after the photon beam 607 passes through thecontainer 606 and its contents. The beam dump 614 and optionaltransmitted flux monitor, detector 609, may be shielded from direct viewof the detectors as required. Signals from the detectors 609, 610, 611,612, and 613 are connected by way of connections 615 to signalprocessing electronics and/or computer 616, which process the detectorsignals and optionally may relay them and/or processed information byway of connections 617 to a central control and data analysis andstorage system (not shown.) Alternatively, the detector signals may bepassed directly to the central system for processing and analysis.

As an alternative to determining neutron energy directly in the neutrondetector, a low duty cycle LINAC (e.g. Varian linatron) or othersuitable electron accelerator may be pulsed to permit a time of flight(TOF) technique. Compared to other detection techniques, such as pulseshape discrimination using a continuous incident photon beam, the TOFmethod is expected to have a higher efficiency for collecting highenergy neutrons, reduced environmental background, and a higherlikelihood of determining angular distributions. The TOF method may usea shortened pulse structure (10 ns) and gated detectors to reject gammaflash. The advantages inherent in the TOF method, combined with themodified LINAC and detectors, may partially compensate for the reducedduty cycle of commonly deployed pulsed accelerators.

In a time-of-flight (TOF) embodiment, the electron accelerator 601 orother source may be pulsed to produce electron beam 602 (pulsed on) fora time period T and turned off for a time long enough to have all thedetectable neutrons (resulting from interactions of the photon beam 607with the container 606 and its contents) pass through the detector(s).Then the electron beam 602 may be pulsed on again for a time period T.This sequence may be repeated until the desired detection data isobtained.

The electron accelerator 601 or some subsidiary target (not shown) nearthe bremsstrahlung target 603 or in the bremsstrahlung or photon beam607 may provide a fiducial signal that informs the signal processingelectronics and/or computer 616 when the photon beam 607 was generated.Neutrons generated by photofission in the fissile sample 608 travel to adetector in the time L/v where L is the distance from the fissile sample608 to the detector in question and v is the neutron velocity. Fordetector 611, for example, which is opposite the fissile sample 608 at aright angle to the incident photon beam 607 in the embodiment shown,L=L611+y, the distance from the fissile sample 608 to the correspondingwall of the container 606 nearest detector 611. The velocity of theneutrons is given by v=(2E/m)^(1/2), where E is the neutron kineticenergy and m is the neutron mass. The signal from detector 611 goes tothe signal processing electronics and/or computer 616, which convertsthe difference between the fiducial signal arrival time and the detector611 signal arrival time into the time-of-flight (TOF) of the neutron tothe detector. Using the relation TOF=(L611+y)/v, the signal processingelectronics and/or computer 616 calculates the neutron velocity andtherefore its energy (E=mv²/2) and records the data and also transfersit to a central control and analysis system (not shown).

The energy resolution of the detection system will depend on the TOF ofthe neutrons, T, L and the dispersion of the flight distance todifferent portions of the detectors. Those experienced in the art willrecognize that these parameters, including the electron beam pulse widthT, and the geometry of the system can be adjusted to achieve energyresolution adequate for the purposes of this disclosure.

The (narrow) photon beam 607 may be scanned across the container 606sequentially to illuminate discrete columns where the fissile sample 608may be located. This serves to better localize the position of anyfissionable material and will reduce backgrounds from other neutronproducing materials in a container. Alternatively, the photon beam 607may be a wide fan-like beam encompassing a greater region of thecontainer 606 with the fan opening out in the direction toward thedetectors at 90 degrees, for example. This allows a broad scan region ofthe container but limited in the narrow direction. Such an embodimentwould facilitate scanning the container in shorter times for fissilematerials. It would detect fissile materials distributed over thedimensions of the fan beam. In this geometry x and y will not be knownbut they may be inferred from a comparison of the neutron energy spectraon both sides of the container since they should be very close toidentical, especially at the highest energies. Starting with anyassumption for “a”, such as ½ the width of the container (x=y), theresulting spectra can be adjusted by varying “a” until the spectra aremade to have the same high-energy shape.

The technology for short duration electron beam pulses is a well-knownart, and pulses of a few nanoseconds are readily generated for highenergy electron beams. Time of flight for a 1 MeV neutron over 1 m is 72nanoseconds. Thus, flight distances of a few meters result in flighttimes (˜71 nanoseconds for 6 MeV neutrons over a distance of 3 meters,for example) that allow beam pulse duration times of 10 to 20nanoseconds to separate photo-fission neutrons from those from (γ, n)processes by energy selection.

Other specific embodiments are possible and some are mentioned herein asfurther illustrations of methods to articulate the concepts and methodsdescribed earlier.

The detectors 610, 611, 612, and 613 in FIG. 6 can be any that arecapable of unambiguously detecting a neutron. Rather than measuring theneutron time of flight to determine its energy, it would suffice in someapplications to only specify that the event is definitely a neutron andthat the energy is greater than a defined amount. This wouldcharacterize the neutron energy as above a defined quantity. Severalsuch neutron energies may be involved. Together with control of theelectron beam energy or photon energy as discussed above, determiningthe number of neutrons with energies above certain preset quantitieswill classify the neutrons as from photo-fission. As discussed above,other processes such as (γ, n) will not be possible at neutron energiesgreater than E=E_(b)−E_(th), where E_(b) is the bremsstrahlung endpointor the photon energy and E_(th) is the threshold for (γ, n) for relevantnon-actinide materials that may be present and need to be distinguishedfrom the suspected actinide.

As discussed above, the energy distribution of neutrons fromphoto-fission is very independent of the energy of the photons used toinduce photo-fission in the photon energy regions discussed herein, inor below the Giant Electric Dipole Resonance. Another embodiment usesthis fact to determine whether the neutrons originate fromphoto-fission. Varying the photon energy or the bremsstrahlung endpointenergy will not substantially alter the energy distribution of theneutrons from photo-fission. However, this is not true for otherprocesses such as (γ, n), especially in the higher regions of neutronenergy, as a result of energy conservation and the requirementE=E_(b)−E_(th), discussed earlier. Therefore, measuring the energydistribution of the neutrons for different photon energies, andcomparing the results, can identify actinide photo-fission.Alternatively, measuring and comparing the number of neutrons above acertain energy as the photon energy is changed can achieve the sameresult.

Another embodiment would measure the neutron yield at a given neutronenergy, as the photon energy is varied, and would do this for severalneutron energies. This would generate yield curves for neutrons of thegiven energies as a function of photon energy. Because the neutronenergy spectra from photon-induced fission is independent of theincident photon energy, the same yield curve as a function of photonenergy would result for all neutron energies if the spectrum isdominated by photo-fission. However, if the neutron spectrum originatesfrom (γ, n) for relevant non-actinide materials, each neutron energy hasa yield curve as a function of photon energy displaced in photon energyby that explicit neutron separation energy, in particular for theneutrons at the highest energy possible. Once again this follows fromenergy conservation.

Neutron detection can be based on reaction energies between the neutronsand the component materials in the detector. Detectors of such a naturemay sometimes but not always be called “threshold detectors” because areaction will occur only if the neutron energy is greater than a certainamount. Examples of such reactions include but are not limited to (n,n′γ), (n, n′f), (n, n′p), (n, n′d) and (n, n′α). Detection of the eventmay be based on, but not limited to, the detection of: a scintillationevent and measuring the deposited energy; the charge created byionization in a material and measuring the total charge; and, thedetection of radioactive nuclei, wherein the radioactivity would beinduced only if the neutron energy (energies) were greater than acertain value (or values). All such methods are included in theembodiments described in this disclosure.

As discussed above, some commercially available plastic and liquidscintillators can identify neutrons unambiguously using suitable signalprocessing techniques. Such detectors also have fast enough timeresponse to qualify for the purposes herein and these will be known tothose skilled in the art. Such detectors operate in part as protonrecoil detectors, based on the energy imparted to protons by the elasticscattering of neutrons from the protons in the hydrogenous material.Therefore, in part, they can function as “threshold detectors” asdiscussed above, as well as providing the time for an event in adetector and identifying the event as a neutron. Such detection methodsare part of the embodiments described herein.

Delayed neutrons following beta decay can also be detected by themethods discussed herein and serve as a method of detecting fissilematerials. They will be less abundant than prompt neutrons by a verylarge factor, as discussed above. In most cases their presence can beused as a further detection method to augment the embodiments discussedherein. They can be distinguished from prompt neutrons by severaltechniques. Using TOF with a pulsed beam set to measure prompt neutrons,delayed neutrons appear as a uniform distribution in time that builds upwith exposure time or the number of pulses in the TOF embodimentdiscussed above. The time for buildup of the delayed neutron signal ischaracteristic of beta-decay lifetimes. If the beam is turned off theywill diminish in times characteristic of beta-decay lifetimes. Thepresence of the delayed neutrons may be neglected in many situations asa minor contribution. In some cases they may be used as an aid to thedetection of fissile material. In all situations, the presence ofdelayed neutrons may be accounted for and the results correctedaccordingly if the correction is required by these embodiments.

The photon beams may be of the pulsed variety described above indiscussing TOF embodiments, or they may be of continuous character asfrom continuous duty radiofrequency accelerators, DC accelerators orsimilarly functioning photon sources of a monochromatic or nearlymonochromatic nature.

Another scan embodiment would employ a very broad beam geometry in alldirections transverse to the beam direction with collimation so as tolimit the beam size to that of the container width in its largestmanifestation. This embodiment would be very effective in the detectionof fissile materials dispersed in small samples over a large volume,such as thin sheets broadly distributed over a large region of thecontainer or small pellets broadly distributed.

Many beam geometries are possible, each with specific advantages forcertain situations as will be recognized by those skilled in the art,and they are all included in this disclosure.

In order to carry out scanning of containers as rapidly as possible, itmay be preferable to carry out an initial scan with a low threshold ortrigger neutron detection energy, in order to maximize the signal fromphotofission, even at the cost of obtaining a signal from (γ, n)processes. If no events are recorded from the container or a portionthereof in an appropriate interval, or no events above an acceptablebackground, the scan can be continued to a further portion of thecontainer, or the container can be passed on if th entire container hasbeen scanned. If events are detected, the threshold or trigger neutrondetection energy can be increased, and the container or portion thereofrescanned, using the higher neutron threshold or trigger detectionenergy to reduce or eliminate the contamination from the competing (γ,n) processes. Alternatively, of course, other of the methods set forthherein for discriminating between photofission and (γ, n) processes canbe employed in the rescan.

Because angular distributions may be difficult to measure given thedifferential absorption and scattering of different cargo loadings, itis important to recognize that, as discussed above, if the energydistribution of the prompt neutrons is independent of angle relative tothe photon beam, then the fragments are emitted isotropically and thefissile material is an odd-even isotope: however, if the prompt neutronshave a spectrum with greater population at the higher energies at 90degrees to the photon beam relative to the prompt neutron spectrum atlarge angles near 180 degrees, then the fragments have an angulardistribution peaking at 90 degrees and the fissile material is aneven-even isotope. Therefore, measuring the neutron energy distributionat two angles will enable this determination to be made.

Another embodiment removes the uncertainty in the energy distributionand angular dependencies of the prompt neutrons caused by thedifferential absorption along different paths that neutrons take intraversing a container to the different detectors. This embodimentdirects the photon beam into the container in different directions. Forexample, in one arrangement the photon beam may enter the container fromthe top and the neutron detectors view the neutrons at 100 degrees tothe beam and at 170 degrees from the beam. By altering the photon beamdirection to enter from the side of the container the detectors changeroles. That one previously at 100 degrees is now at 170 degrees and theone previously at 170 degrees is now at 100 degrees. However, thedifferential aspects of neutron absorption remain exactly the same. Thetwo measurements now provide a clear indication of the influence on theneutron energy distribution of the angle of emission of the neutronrelative to the photon direction as well as the angular distribution ofthe neutrons relative to the photon beam direction. As one particularfeature, if the photo-fission process is isotropic the relative neutronyields in the detectors will not change. A change indicates anisotropyin the original photo-fission process.

This process can be generalized for other angles as well. For example,FIGS. 8A and 8B, each show a container 806 with fissile material 808. Afirst neutron detector 801 is shown in a first location and a secondneutron detector 802 is shown in a second location. In FIG. 8A, a photonbeam 807A irradiates the container 806 from a first direction (direction1). In FIG. 8B, a photon beam 807B irradiates the container 806 from asecond direction (direction 2) For each of the two photon beamdirections, the neutron detectors 801 and 802 interchange anglesrelative to the photon beam (807A or 807B) direction. For beam direction1, first neutron detector 801 is at angle θ₁ and second neutron detector802 is at angle θ₂. For beam direction 2, first neutron detector 801 isat angle θ₂ and second neutron detector 802 is at angle θ₁. I₁ and I₂are the photon beam intensities at the target 808 (which may be afissile material) for photon beam directions 1 and 2 respectively. IfS(E,θ) is the energy spectrum of neutrons produced in direction θ, theneutrons detected by the two detectors with photon beam direction 1 aredescribed by the measured functions F_(i)(E, θ_(j));

-   F₁(E,θ₁)=I₁×A₁(E)×S(E,θ₁) for first neutron detector 801; and,-   F₂(E,θ₂)=I₁×A₂(E)×S(E,θ₂) for second neutron detector 802.

The neutrons detected by the two detectors with beam in direction 2 are:

-   F₁(E,θ₂)=I₂×A₁(E)×S(E,θ₂) for first neutron detector 801; and,-   F₂(E,θ₁)=I₂×A₂(E)×S(E,θ₁) for second neutron detector 802.

The attenuation factors A₁ and A₂ remain invariant to the beam positionand the ratio can be formed to eliminate these factors so that:

{S(E,θ ₁)/S(E,θ ₂)}² ={F ₁(E,θ ₁)×F ₂(E,θ ₁)}/{F ₂(E,θ ₂)×F ₁(E,θ ₂)}.   (Equation 1)

Thus, S(E,θ₁) and S(E,θ₂) are related via measured quantities and can becompared directly. A person skilled in the art will be able generalizethis technique to more than two detectors and this embodiment isintended to contain all these variations.

Unless otherwise specified, the illustrative embodiments can beunderstood as providing exemplary features of varying detail of certainembodiments, and therefore, unless otherwise specified, features,components, modules, and/or aspects of the embodiments can be otherwisecombined, specified, interchanged, and/or rearranged without departingfrom the disclosed devices or methods. Additionally, the shapes andsizes of components are also exemplary, and unless otherwise specified,can be altered without affecting the disclosed devices or methods. Otherspecific embodiments are possible and some are mentioned herein asfurther illustrations of methods to articulate the concepts and methodsdescribed earlier.

Although the terms “nuclear material”, “fissionable nuclear material”,“fissile material”, and “fissionable material” have been variously usedin this disclosure, the intent of the inventors is that these terms areused interchangeably and are all intended to designate those materialsthat can be induced to fission by the effect of a gamma ray or by athermal neutron or fast neutron. These terms are not intended to meanmaterials that emit neutrons in response to gamma or neutronirradiation, unless such materials also may be induced to fission by theeffect of a gamma ray or by a thermal neutron or a fast neutron. Theterm “container” as used herein is intended to include any enclosure orpartial enclosure that may enclose or partially enclose a fissionablematerial so as to hide or partly hide it or shield it or partly shieldit from conventional detection methods—it includes but is not limited tocargo and shipping containers and vehicles.

While the systems and methods disclosed herein have been particularlyshown and described with references to exemplary embodiments thereof, itwill be understood by those skilled in the art that various changes inform and details may be made therein without departing from the scope ofthe disclosure. It should be realized this disclosure is also capable ofa wide variety of further and other embodiments within the spirit of thedisclosure. Those skilled in the art will recognize or be able toascertain using no more than routine experimentation, many equivalentsto the exemplary embodiments described specifically herein. Suchequivalents are intended to be encompassed in the scope of the presentdisclosure.

1. A method of detecting a presence of a material comprising an actinidein a container, comprising: a) locating the container such that at leastone neutron detector views the said container at a first angle relativeto the photon beam; b) for at least two predetermined cutoff photonenergies, illuminating at least a portion of the said container with aphoton beam comprising photons of energies no greater than the saidpredetermined cutoff photon energy; c) for at least two of saidpredetermined cutoff photon energies, detecting in at least one saidneutron detector at least some neutrons produced by an interaction ofthe said photon beam with at least a portion of the said container; d)for at least two of said predetermined cutoff photon energies, for eachof a plurality of said detected neutrons, determining the energy of thesaid detected neutron; e) choosing a higher neutron energy region whereneutrons from a (γ, n) process are not energetically permitted for anyof said at least two predetermined cutoff photon energies, and a lowerenergy region where neutrons from a (γ, n) process are energeticallypermitted for all of said at least two predetermined cutoff photonenergies; f) for at least two of said predetermined cutoff photonenergies, determining a neutron yield in one of said neutron detectorsin at least two predetermined neutron energy ranges, wherein at leastone predetermined neutron energy range encompasses the higher energyregion where neutrons from a (γ, n) process are not energeticallypermitted; and wherein at least one other predetermined neutron energyrange encompasses the lower energy region where neutrons from a (γ, n)process are energetically permitted; and g) based upon comparing thesaid determined neutron yields in the said one neutron detector,resulting from the said incident photon beams comprising photons ofenergies no greater than said different predetermined cutoff energies,confirming that the material comprising the present actinide is presentin the container if an increase in the said neutron yield between alower predetermined cutoff photon energy and a higher predeterminedcutoff photon energy, in the higher predetermined neutron energy rangewhere neutrons from a (γ, n) process are not energetically permitted, isnot substantial in comparison to an increase in neutron yield in thelower predetermined neutron energy range, where neutrons from a (γ, n)process are energetically permitted.
 2. The method of claim 1, whereinthe photon beam comprising photons of energies no greater than a saidpredetermined cutoff photon energy is a bremsstrahlung beam produced byelectrons of the said predetermined cutoff energy.
 3. The method ofclaim 1, wherein the photon beam comprising photons of energies nogreater than a said predetermined cutoff photon energy is amonochromatic photon beam.
 4. The method of claim 1, wherein determiningthe energy of the said detected neutron comprises measuring a time offlight of the said detected neutron.
 5. The method of claim 1, whereindetermining the energy of the said detected neutron comprises analyzingthe energy deposited in at least one of said neutron detectors.
 6. Themethod of claim 1, wherein the container is located such that at leasttwo neutron detectors view the said container, the said neutrondetectors viewing the container from different angles relative to thephoton beam, and neutrons are detected in at least two said neutrondetectors; further comprising: h) for at least one of said predeterminedcutoff photon energies, determining a total neutron yield in at leasttwo of said neutron detectors in a predetermined neutron energy range;and i) based upon comparing the said total neutron yields from the saidat least two neutron detectors viewing the container from differentangles relative to the photon beam, determining that the presentactinide is an odd-even isotope if the total yields disclose anisotropic distribution of neutrons as a function of angle relative tothe photon beam, and determining that the present actinide is aneven-even isotope if the total yields disclose an anisotropicdistribution of neutrons as a function of angle relative to the photonbeam.
 7. The method of claim 1, further comprising: h) for at least onenew angle relative to the photon beam, i) moving at least one of saidneutron detectors such that the said moved neutron detector views thesaid container from the said new angle relative to the photon beam; ii)illuminating at least a portion of the said container with the photonbeam comprising photons of energies no greater than at least one of saidpredetermined cutoff photon energies; iii) detecting in said at leastone moved neutron detector at least some neutrons produced by aninteraction of the said photon beam with at least a portion of the saidcontainer; and iv) for each of a plurality of said detected neutrons,determining the energy of the said detected neutron; i) for at leastsome of said neutron detector viewing angles relative to the photonbeam, determining a total neutron yield in the said at least one movedneutron detector in a predetermined neutron energy range at the saidangle relative to the photon beam; and j) based upon comparing the saidtotal neutron yields, from the said at least one moved neutron detector,for at least some of the neutron detector viewing angles relative to thephoton beam, determining that the present actinide is an odd-evenisotope if the total yields disclose an isotropic distribution ofneutrons as a function of angle relative to the photon beam, anddetermining that the present actinide is an even-even isotope if thetotal yields disclose an anisotropic distribution of neutrons as afunction of angle relative to the photon beam.
 8. The method of claim 1,wherein the container is located such that at least two neutrondetectors view the said container, the said neutron detectors viewingthe container from different angles relative to the photon beam, andneutrons are detected in at least two said neutron detectors; furthercomprising: h) for at least one of said plurality of predeterminedcutoff photon energies, determining a neutron energy distribution in atleast two of said neutron detectors; and i) based upon comparing thesaid neutron energy distributions from the said at least two neutrondetectors viewing the container from different angles relative to thephoton beam, determining that the present actinide is an odd-evenisotope if the energy distributions do not change by more than apredetermined amount as a function of angle relative to the photon beam,and determining that the present actinide is an even-even isotope if theenergy distributions change by more than a predetermined amount as afunction of angle relative to the photon beam.
 9. The method of claim 1,further comprising: h) for at least one new angle relative to the photonbeam, i) moving at least one of said neutron detectors such that thesaid moved neutron detector views the said container from the said newangle relative to the photon beam; ii) illuminating at least a portionof the said container with the photon beam comprising photons ofenergies no greater than at least one of said predetermined cutoffphoton energies; iii) detecting in said at least one moved neutrondetector at least some neutrons produced by an interaction of the saidphoton beam with at least a portion of the said container; and iv) foreach of a plurality of said detected neutrons, determining the energy ofthe said detected neutron; i) for at least some of said neutron detectorviewing angles relative to the photon beam, determining a neutron energydistribution in the said at least one moved neutron detector; and j)based upon comparing the said neutron energy distributions, from thesaid at least one moved neutron detector, for at least some of theneutron detector viewing angles relative to the photon beam, determiningthat the present actinide is an odd-even isotope if the energydistributions do not change by more than a predetermined amount as afunction of angle relative to the photon beam, and determining that thepresent actinide is an even-even isotope if the energy distributionschange by more than a predetermined amount as a function of anglerelative to the photon beam.
 10. A method of detecting a presence of amaterial comprising an actinide in a container, comprising: a) locatingthe container such that at least one neutron detector views the saidcontainer at a first angle relative to the photon beam; b) for at leasttwo predetermined cutoff photon energies, illuminating at least aportion of the said container with a photon beam comprising photons ofenergies no greater than the said predetermined cutoff photon energy; c)for at least two of said predetermined cutoff photon energies, detectingin at least one said neutron detector at least some neutrons produced byan interaction of the said photon beam with at least a portion of thesaid container; d) for at least two of said predetermined cutoff photonenergies, for each of a plurality of said detected neutrons, determiningthe energy of the said detected neutron; e) for at least two of saidpredetermined cutoff photon energies, determining a neutron energydistribution in one of said neutron detectors; and f) based uponcomparing the said determined neutron energy distributions in the saidone neutron detector, resulting from the said incident photon beamscomprising photons of energies no greater than said at least twopredetermined cutoff photon energies, identifying the materialcomprising the actinide as present in the container if the said neutronenergy distributions change by no more than a predetermined amount as afunction of cutoff photon energy.
 11. The method of claim 10, whereinthe photon beam comprising photons of energies no greater than a saidpredetermined cutoff photon energy is a bremsstrahlung beam produced byelectrons of the said predetermined cutoff energy.
 12. The method ofclaim 10, wherein the photon beam comprising photons of energies nogreater than a said predetermined cutoff photon energy is amonochromatic photon beam.
 13. The method of claim 10, whereindetermining the energy of the said detected neutron comprises measuringa time of flight of the said detected neutron.
 14. The method of claim10, wherein determining the energy of the said detected neutroncomprises analyzing the energy deposited in at least one of said neutrondetectors.
 15. The method of claim 10, wherein the container is locatedsuch that at least two neutron detectors view the said container, thesaid neutron detectors viewing the container from different anglesrelative to the photon beam, and neutrons are detected in at least twosaid neutron detectors; further comprising: g) for at least one of saidpredetermined cutoff photon energies, determining a total neutron yieldin at least two of said neutron detectors in a predetermined neutronenergy range; and h) based upon comparing the said total neutron yieldsfrom the said at least two neutron detectors viewing the container fromdifferent angles relative to the photon beam, determining that thepresent actinide is an odd-even isotope if the total yields disclose anisotropic distribution of neutrons as a function of angle relative tothe photon beam, and determining that the present actinide is aneven-even isotope if the total yields disclose an anisotropicdistribution of neutrons as a function of angle relative to the photonbeam.
 16. The method of claim 10, further comprising: g) for at leastone new angle relative to the photon beam, i) moving at least one ofsaid neutron detectors such that the said moved neutron detector viewsthe said container from the said new angle relative to the photon beam;ii) illuminating at least a portion of the said container with thephoton beam comprising photons of energies no greater than at least oneof said predetermined cutoff photon energies; iii) detecting in said atleast one moved neutron detector at least some neutrons produced by aninteraction of the said photon beam with at least a portion of the saidcontainer; and iv) for each of a plurality of said detected neutrons,determining the energy of the said detected neutron; h) for at leastsome of said neutron detector viewing angles relative to the photonbeam, determining a total neutron yield in the said at least one movedneutron detector in a predetermined neutron energy range at the saidangle relative to the photon beam; and i) based upon comparing the saidtotal neutron yields, from the said at least one moved neutron detector,for at least some of the neutron detector viewing angles relative to thephoton beam, determining that the present actinide is an odd-evenisotope if the total yields disclose an isotropic distribution ofneutrons as a function of angle relative to the photon beam, anddetermining that the present actinide is an even-even isotope if thetotal yields disclose an anisotropic distribution of neutrons as afunction of angle relative to the photon beam.
 17. The method of claim10, wherein the container is located such that at least two neutrondetectors view the said container, the said neutron detectors viewingthe container from different angles relative to the photon beam, andneutrons are detected in at least two said neutron detectors; furthercomprising: g) for at least one of said plurality of predeterminedcutoff photon energies, determining a neutron energy distribution in atleast two of said neutron detectors; and h) based upon comparing thesaid neutron energy distributions from the said at least two neutrondetectors viewing the container from different angles relative to thephoton beam, determining that the present actinide is an odd-evenisotope if the energy distributions do not change by more than apredetermined amount as a function of angle relative to the photon beam,and determining that the present actinide is an even-even isotope if theenergy distributions change by more than a predetermined amount as afunction of angle relative to the photon beam.
 18. The method of claim10, further comprising: g) for at least one new angle relative to thephoton beam, i) moving at least one of said neutron detectors such thatthe said moved neutron detector views the said container from the saidnew angle relative to the photon beam; ii) illuminating at least aportion of the said container with the photon beam comprising photons ofenergies no greater than at least one of said predetermined cutoffphoton energies; iii) detecting in said at least one moved neutrondetector at least some neutrons produced by an interaction of the saidphoton beam with at least a portion of the said container; and iv) foreach of a plurality of said detected neutrons, determining the energy ofthe said detected neutron; h) for at least some of said neutron detectorviewing angles relative to the photon beam, determining a neutron energydistribution in the said at least one moved neutron detector; and i)based upon comparing the said neutron energy distributions, from thesaid at least one moved neutron detector, for at least some of theneutron detector viewing angles relative to the photon beam, determiningthat the present actinide is an odd-even isotope if the energydistributions do not change by more than a predetermined amount as afunction of angle relative to the photon beam, and determining that thepresent actinide is an even-even isotope if the energy distributionschange by more than a predetermined amount as a function of anglerelative to the photon beam.
 19. A method of detecting a presence of amaterial comprising an actinide in a container, comprising: a) locatingthe container such that at least one neutron detector views the saidcontainer at a first angle relative to the photon beam; b) for at leasttwo predetermined cutoff photon energies, illuminating at least aportion of the said container with a photon beam comprising photons ofenergies no greater than the said predetermined cutoff photon energy; c)for at least two of said predetermined cutoff photon energies, detectingin at least one said neutron detector at least some neutrons produced byan interaction of the said photon beam with at least a portion of thesaid container; d) for at least two of said predetermined cutoff photonenergies, for each of a plurality of said detected neutrons, determiningthe energy of the said detected neutron; e) for at least two of saidpredetermined cutoff photon energies, determining a neutron yield in oneof said neutron detectors in a plurality of predetermined neutron energyranges; f) for each said predetermined neutron energy range; determininga neutron yield curve as a function of photon cutoff energy; and g)based upon comparing the said determined neutron yield curves in thesaid one neutron detector for the said predetermined neutron energyranges, identifying the material comprising the actinide as present inthe container if the said neutron yield curves change by no more than apredetermined amount as a function of neutron energy.
 20. The method ofclaim 19, wherein the photon beam comprising photons of energies nogreater than a said predetermined cutoff photon energy is abremsstrahlung beam produced by electrons of the said predeterminedcutoff energy.
 21. The method of claim 19, wherein the photon beamcomprising photons of energies no greater than a said predeterminedcutoff photon energy is a monochromatic photon beam.
 22. The method ofclaim 19, wherein determining the energy of the said detected neutroncomprises measuring a time of flight of the said detected neutron. 23.The method of claim 19, wherein determining the energy of the saiddetected neutron comprises analyzing the energy deposited in at leastone of said neutron detectors.
 24. The method of claim 19, wherein thecontainer is located such that at least two neutron detectors view thesaid container, the said neutron detectors viewing the container fromdifferent angles relative to the photon beam, and neutrons are detectedin at least two said neutron detectors; further comprising: h) for atleast one of said predetermined cutoff photon energies, determining atotal neutron yield in at least two of said neutron detectors in apredetermined neutron energy range; and i) based upon comparing the saidtotal neutron yields from the said at least two neutron detectorsviewing the container from different angles relative to the photon beam,determining that the present actinide is an odd-even isotope if thetotal yields disclose an isotropic distribution of neutrons as afunction of angle relative to the photon beam, and determining that thepresent actinide is an even-even isotope if the total yields disclose ananisotropic distribution of neutrons as a function of angle relative tothe photon beam.
 25. The method of claim 19, further comprising: h) forat least one new angle relative to the photon beam, i) moving at leastone of said neutron detectors such that the said moved neutron detectorviews the said container from the said new angle relative to the photonbeam; ii) illuminating at least a portion of the said container with thephoton beam comprising photons of energies no greater than at least oneof said predetermined cutoff photon energies; iii) detecting in said atleast one moved neutron detector at least some neutrons produced by aninteraction of the said photon beam with at least a portion of the saidcontainer; and iv) for each of a plurality of said detected neutrons,determining the energy of the said detected neutron; i) for at leastsome of said neutron detector viewing angles relative to the photonbeam, determining a total neutron yield in the said at least one movedneutron detector in a predetermined neutron energy range at the saidangle relative to the photon beam; and j) based upon comparing the saidtotal neutron yields, from the said at least one moved neutron detector,for at least some of the neutron detector viewing angles relative to thephoton beam, determining that the present actinide is an odd-evenisotope if the total yields disclose an isotropic distribution ofneutrons as a function of angle relative to the photon beam, anddetermining that the present actinide is an even-even isotope if thetotal yields disclose an anisotropic distribution of neutrons as afunction of angle relative to the photon beam.
 26. The method of claim19, wherein the container is located such that at least two neutrondetectors view the said container, the said neutron detectors viewingthe container from different angles relative to the photon beam, andneutrons are detected in at least two said neutron detectors; furthercomprising: h) for at least one of said plurality of predeterminedcutoff photon energies, determining a neutron energy distribution in atleast two of said neutron detectors; and i) based upon comparing thesaid neutron energy distributions from the said at least two neutrondetectors viewing the container from different angles relative to thephoton beam, determining that the present actinide is an odd-evenisotope if the energy distributions do not change by more than apredetermined amount as a function of angle relative to the photon beam,and determining that the present actinide is an even-even isotope if theenergy distributions change by more than a predetermined amount as afunction of angle relative to the photon beam.
 27. The method of claim19, further comprising: h) for at least one new angle relative to thephoton beam, i) moving at least one of said neutron detectors such thatthe said moved neutron detector views the said container from the saidnew angle relative to the photon beam; ii) illuminating at least aportion of the said container with the photon beam comprising photons ofenergies no greater than at least one of said predetermined cutoffphoton energies; iii) detecting in said at least one moved neutrondetector at least some neutrons produced by an interaction of the saidphoton beam with at least a portion of the said container; and iv) foreach of a plurality of said detected neutrons, determining the energy ofthe said detected neutron; i) for at least some of said neutron detectorviewing angles relative to the photon beam, determining a neutron energydistribution in the said at least one moved neutron detector; and j)based upon comparing the said neutron energy distributions, from thesaid at least one moved neutron detector, for at least some of theneutron detector viewing angles relative to the photon beam, determiningthat the present actinide is an odd-even isotope if the energydistributions do not change by more than a predetermined amount as afunction of angle relative to the photon beam, and determining that thepresent actinide is an even-even isotope if the energy distributionschange by more than a predetermined amount as a function of anglerelative to the photon beam.
 28. A method of detecting a presence of amaterial comprising an actinide in a container, comprising: a) locatingthe container such that at least one neutron detector views the saidcontainer; b) for at least two predetermined cutoff photon energies,illuminating at least a portion of the said container with a photon beamcomprising photons of energies no greater than the said predeterminedcutoff photon energy; c) for at least two of said predetermined cutoffphoton energies, detecting in at least one said neutron detector atleast some neutrons produced by an interaction of the said photon beamwith at least a portion of the said container; d) for at least two ofsaid predetermined cutoff photon energies, for each of a plurality ofsaid detected neutrons, determining the minimum energy of the saiddetected neutron; e) choosing a higher neutron energy region whereneutrons from a (γ, n) process are not energetically permitted for anyof the at least two of said predetermined cutoff photon energies, and alower energy region where neutrons from a (γ, n) process areenergetically permitted for all of the at least two said predeterminedcutoff photon energies; f) for at least two of said predetermined cutoffphoton energies, determining a neutron yield in one of said neutrondetectors in at least two predetermined neutron minimum energy ranges,wherein at least one predetermined neutron minimum energy rangeencompasses the higher energy region where neutrons from a (γ, n)process are not energetically permitted; and wherein at least one otherpredetermined neutron minimum energy range encompasses the lower energyregion where neutrons from a (γ, n) process are energetically permitted;and g) based upon comparing the said determined neutron yields in thesaid one neutron detector, resulting from the said incident photon beamscomprising photons of energies no greater than said differentpredetermined cutoff energies, confirming that the material comprisingthe present actinide is present in the container if an increase in thesaid neutron yield between a lower predetermined cutoff photon energyand a higher predetermined cutoff photon energy, in the higherpredetermined neutron minimum energy range where neutrons from a (γ, n)process are not energetically permitted, is not substantial incomparison to an increase in neutron yield in the lower predeterminedneutron minimum energy range, where neutrons from a (γ, n) process areenergetically permitted.
 29. A method of detecting a presence of amaterial comprising an actinide in a container, comprising: a) locatingthe container such that at least one neutron detector views the saidcontainer; b) for at least two predetermined cutoff photon energies,illuminating at least a portion of the said container with a photon beamcomprising photons of energies no greater than the said predeterminedcutoff photon energy; c) for at least two of said predetermined cutoffphoton energies, detecting in at least one said neutron detector atleast some neutrons produced by an interaction of the said photon beamwith at least a portion of the said container; d) for at least two ofsaid predetermined cutoff photon energies, for each of a plurality ofsaid detected neutrons, determining the minimum energy of the saiddetected neutron; e) for at least two of said predetermined cutoffphoton energies, determining a neutron minimum energy distribution inone of said neutron detectors; and f) based upon comparing the saiddetermined neutron minimum energy distributions in the said one neutrondetector, resulting from the said incident photon beams comprisingphotons of energies no greater than said at least two predeterminedcutoff photon energies, identifying the material comprising the actinideas present in the container if the said neutron minimum energydistributions change by no more than a predetermined amount as afunction of cutoff photon energy.
 30. A method of detecting a presenceof a material comprising an actinide in a container, comprising: a)locating the container such that at least one neutron detector views thesaid container; b) for at least two predetermined cutoff photonenergies, illuminating at least a portion of the said container with aphoton beam comprising photons of energies no greater than the saidpredetermined cutoff photon energy; c) for at least two of saidpredetermined cutoff photon energies, detecting in at least one saidneutron detector at least some neutrons produced by an interaction ofthe said photon beam with at least a portion of the said container; d)for at least two of said predetermined cutoff photon energies, for eachof a plurality of said detected neutrons, determining the minimum energyof the said detected neutron; e) for at least two of said predeterminedcutoff photon energies, determining a neutron yield in one of saidneutron detectors in a plurality of predetermined neutron minimum energyranges; f) for each said predetermined neutron minimum energy range;determining a neutron yield curve as a function of photon cutoff energy;and g) based upon comparing the said determined neutron yield curves inthe said one neutron detector for the said predetermined neutron minimumenergy ranges, identifying the material comprising the actinide aspresent in the container if the said neutron yield curves change by nomore than a predetermined amount as a function of neutron energy.